Add rustc_never_type_mode crate-level attribute to allow experimenting

compiler/rustc_span/src/symbol.rs

@@ -815,6 +815,8 @@ symbols! {
         fadd_algebraic,
         fadd_fast,
         fake_variadic,
+        fallback_to_niko,
+        fallback_to_unit,
         fdiv_algebraic,
         fdiv_fast,
         feature,
@@ -1233,6 +1235,7 @@ symbols! {
         no_crate_inject,
         no_debug,
         no_default_passes,
+        no_fallback,
         no_implicit_prelude,
         no_inline,
         no_link,
@@ -1551,6 +1554,7 @@ symbols! {
         rustc_mir,
         rustc_must_implement_one_of,
         rustc_never_returns_null_ptr,
+        rustc_never_type_mode,
         rustc_no_mir_inline,
         rustc_nonnull_optimization_guaranteed,
         rustc_nounwind,